Sun
The Sun formed about 4.6 billion years ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and helium. A shock wave from a nearby supernova likely triggered this formation by compressing matter within the cloud. This event caused certain regions to collapse under their own gravity, creating the star we see today. Studies of ancient meteorites reveal traces of stable daughter nuclei of short-lived isotopes like iron-60. These isotopes form only in exploding stars, confirming that one or more supernovae occurred near where the Sun was born. The resulting protoplanetary disk became the planets and other Solar System bodies. The Sun is now roughly halfway through its main-sequence stage. It has not changed dramatically in over four billion years and will remain fairly stable for about five billion more. Each second, more than four billion kilograms of matter are converted into energy within the core. At this rate, the Sun has so far converted around 100 times the mass of Earth into energy. In approximately 5 billion years, core hydrogen fusion will stop. The release of gravitational potential energy will cause the luminosity of the Sun to increase. The Sun will expand over the next billion years first into a subgiant and then into a red giant. When it enters the red-giant branch phase, it will engulf Mercury and Venus. Models suggest Earth's orbit will initially expand but eventually shrink due to tidal forces. Earth will be engulfed by the Sun during the tip of the red-giant branch phase 7.59 billion years from now. After the red-giant branch, the Sun has approximately 120 million years of active life left. The core full of degenerate helium ignites violently in the helium flash. This process converts 6% of the core into carbon within minutes. The Sun then shrinks to around 10 times its current size and 50 times the luminosity. It reaches the red clump or horizontal branch before becoming moderately larger and more luminous over about 100 million years. When helium is exhausted, the Sun repeats the expansion it followed when hydrogen was depleted. This time however all happens faster as it becomes larger and more luminous. The asymptotic-giant-branch phase sees the Sun alternately reacting hydrogen in a shell or helium in a deeper shell. Thermal pulses become larger each time pushing the luminosity to as much as 5,000 times the current level. The final naked core becomes a white dwarf with a temperature of over 100,000 Kelvin. Simulations indicate that the Sun may be among the least massive stars capable of forming a planetary nebula. The planetary nebula will disperse in about 10,000 years but the white dwarf will survive for trillions of years before fading to a hypothetical super-dense black dwarf.
The core of the Sun extends from the centre to about 20, 25% of the solar radius. It has a density of up to 150 grams per cubic centimeter which is about 150 times the density of water. A temperature close to 15.7 million kelvin exists within this region. By contrast the Sun's surface temperature is about 5,800 kelvin. Through most of the Sun's life energy has been produced by nuclear fusion in the core region through the proton-proton chain. This process converts hydrogen into helium. Currently 0.8% of the energy generated comes from another sequence called the CNO cycle. The proportion coming from the CNO cycle is expected to increase as the Sun becomes older and more luminous. The core is the only region that produces an appreciable amount of thermal energy through fusion. Ninety-nine percent of the power is generated in the innermost 24% of its radius. Almost no fusion occurs beyond 30% of the radius. The rest of the Sun is heated by this energy as it transfers outward through many successive layers. Photons escape the Sun through radiation or advection at the photosphere. Estimates of photon travel time range between 10,000 and 170,000 years. In contrast it takes only 2.3 seconds for neutrinos to reach the surface. Neutrinos account for about 2% of total energy production. Electron neutrinos are released by fusion reactions but almost all escape immediately. Measurements show fewer electron neutrinos than theories predict by a factor of three. In 2001 the discovery of neutrino oscillation resolved this discrepancy. The Sun emits the number predicted but detectors miss two-thirds because they change flavor before detection. The radiative zone starts above the core at about 0.25 solar radii and extends out to 0.7 solar radii. This zone is so named because thermal radiation is the primary means of energy transfer. Photons scatter from dense gas so often that they take millions of years to cross this zone. Temperature drops from approximately 7 million to 2 million kelvins with increasing distance from the core. Density drops a hundredfold between 0.25 and 0.7 solar radii. The tachocline separates the radiative zone and convective zone. A sharp regime change occurs here resulting in large shear between layers. A magnetic dynamo within this layer generates the Sun's magnetic field. The convection zone extends from 0.7 solar radii to near the surface. Material heated at the tachocline picks up heat and expands reducing its density. An orderly motion develops into thermal cells carrying most heat outward. At the photosphere temperature has dropped 350-fold to 5,800 K. Density reaches only 0.2 grams per cubic meter which is one ten-thousandth that of air at sea level. The thermal columns form hexagonal prisms known as Bénard cells. These create granulation on the surface called solar granulation. The photosphere consists mostly of hydrogen at roughly 74% and helium at 23.8%. All heavier elements account for less than 2% of mass. Oxygen makes up about 1% while carbon neon and iron each comprise around 0.2%. The original chemical composition was inherited from the interstellar medium out of which it formed. Originally it would have been about 71.1% hydrogen and 27.4% helium. Over the past 4.6 billion years the amount of helium within the core has increased from about 24% to 60%. Some helium and heavy elements have settled toward the centre due to gravity.
The Sun has a stellar magnetic field that varies across its surface. Its polar field measures approximately 1 gauss whereas the field is typically 1,000 to 4,000 gauss in sunspots. The quasi-periodic 11-year solar cycle is the most prominent variation. Number and size of sunspots wax and wane during this period. Sunspots are visible as dark patches where convective transport of heat is inhibited. They appear slightly cooler than surrounding photosphere so they look dark. At typical solar minimum few sunspots are visible and occasionally none can be seen. Those that do appear form at high solar latitudes. As the cycle progresses toward maximum sunspots tend to form closer to the equator. This phenomenon follows Spörer's law. Largest sunspots can span tens of thousands of kilometers. An 11-year sunspot cycle represents half of a 22-year Babcock, Leighton dynamo cycle. Energy oscillates between toroidal and poloidal magnetic fields. At solar-cycle maximum external poloidal dipolar field nears dynamo-cycle minimum strength. Internal toroidal quadrupolar field reaches maximum strength through differential rotation within the tachocline. Buoyant upwelling forces emergence of toroidal field through photosphere. Pairs of sunspots align east-west with opposite magnetic polarities. Magnetic polarity alternates every solar cycle described by Hale's law. During declining phase energy shifts from internal toroidal field to external poloidal field. Sunspots diminish in number and size. At solar-cycle minimum toroidal field reaches minimum strength while poloidal field peaks. With rise of next 11-year cycle differential rotation shifts energy back from poloidal to toroidal field but with opposite polarity. The process continues continuously. In an idealized scenario each 11-year cycle corresponds to change in overall polarity of large-scale magnetic field. Solar flares and coronal mass ejections occur at sunspot groups. Slowly changing high-speed streams emit from coronal holes at photospheric surface. Both carry plasma and interplanetary magnetic field outward into the Solar System. Effects on Earth include auroras at moderate to high latitudes and disruption of radio communications. Changes in solar irradiance over the 11-year cycle correlate with changes in sunspot number. The solar cycle influences space weather conditions surrounding Earth. In the 17th century the solar cycle appeared to stop entirely for several decades. Few sunspots were observed during a period known as the Maunder minimum. This coincided with the Little Ice Age when Europe experienced unusually cold temperatures. Earlier extended minima discovered through tree ring analysis appear to have coincided with lower-than-average global temperatures.
In many prehistoric and ancient cultures the Sun was thought to be a solar deity or other supernatural entity. One of first people to offer scientific explanation was Greek philosopher Anaxagoras. He reasoned it was giant flaming ball of metal even larger than land of Peloponnesus. Eratosthenes estimated distance between Earth and Sun in 3rd century BC as 4,080,000 stadia or 804,000,000 stadia. Latter value correct within few percent. Ptolemy estimated distance as 1,210 times radius of Earth approximately 8 million kilometers. Theory that Sun is center around which planets orbit proposed by Aristarchus of Samos in 3rd century BC. Nicolaus Copernicus developed detailed mathematical model in 16th century. Observations recorded by Chinese astronomers during Han dynasty from 202 BC to AD 220. Averroes provided description of sunspots in 12th century. Invention of telescope in early 17th century permitted detailed observations by Thomas Harriot and Galileo Galilei. Galileo posited sunspots were on surface rather than small objects passing between Earth and Sun. Medieval Islamic contributions include al-Battani's discovery that direction of Sun's apogee changes gradually. Ibn Yunus observed more than 10,000 entries for Sun's position using large astrolabe. First reasonably accurate distance determined in 1684 by Giovanni Domenico Cassini. He measured Martian parallax sending Jean Richer to Cayenne part of French Guiana. Cassini calculated Earth-Mars distance then used Kepler's laws to determine Earth-Sun distance. His value about 10% smaller than modern values was much larger than all previous estimates. Edmond Halley observed transit of Mercury across Sun in 1677 leading him to realize solar parallax could be trigonometrically determined. Observations of 1769 transit allowed calculation as 150 million kilometers only 0.8% greater than modern value. Isaac Newton observed sunlight using prism showing it made up of many colors. William Herschel discovered infrared radiation beyond red part of spectrum in 1800. Joseph von Fraunhofer recorded more than 600 absorption lines strongest still called Fraunhofer lines. Norman Lockyer hypothesized new element helium after Greek god Helios in 1868. Twenty-five years later helium isolated on Earth. Lord Kelvin suggested Sun is gradually cooling liquid body radiating internal store of heat. Kelvin and Hermann von Helmholtz proposed gravitational contraction mechanism but resulting age estimate only 20 million years well short of geological discoveries suggesting at least 300 million years. Ernest Rutherford suggested radioactive decay as source in 1904. Albert Einstein provided essential clue with mass-energy equivalence relation. Sir Arthur Eddington proposed nuclear fusion reaction merging hydrogen into helium nuclei in 1920. Cecilia Payne confirmed preponderance of hydrogen in 1925 using ionization theory developed by Meghnad Saha. Subrahmanyan Chandrasekhar and Hans Bethe developed theoretical concept of fusion in 1930s. Margaret Burbidge Geoffrey Burbidge William Fowler and Fred Hoyle showed most elements synthesized inside stars in 1957.
The Sun orbits galaxy's center of mass at average speed of 230 kilometers per second which equals 828,000 km/h. It takes about 220 to 250 million Earth years to complete one revolution known as galactic year. The Sun has done so approximately 20 times since its formation. Direction of motion called Solar apex points roughly toward star Vega. In past the Sun likely moved through Orion-Eridanus Superbubble before entering Local Bubble. As Sun goes around galaxy it also moves relative to average motion of other stars. Simple model predicts elliptical circulation around point itself going around galaxy. Period of circulation around that point is about 166 million years shorter than time for point to go around galaxy. Length ellipse around 1,760 parsecs width around 1,170 parsecs. Distance from center of galaxy around 7 or 8 kiloparsecs. At same time Sun moves north and south of galactic plane with different period around 83 million years moving about 99 parsecs away from plane. Point around which Sun circulates takes around 240 million years to go once around galaxy. Orbital perturbation occurs due to non-uniform mass distribution in Milky Way such as spiral arms. Passage through higher density spiral arms often coincides with mass extinctions on Earth perhaps due to increased impact events. Takes Solar System about 225 to 250 million years to complete orbit through Milky Way thought to have completed 20 to 25 orbits during lifetime of Sun. Orbital speed approximately 251 km/s equals 156 miles per second. At this speed takes around 1,190 years to travel distance of one light-year. It takes 7 days to travel 1 astronomical unit. Milky Way moves relative to cosmic microwave background radiation in direction of constellation Hydra at speed of 550 km/s. Since Sun moves relative to Galactic Center in direction of Cygnus at more than 200 km/sec resultant velocity relative to CMB is about 370 km/s in direction of Crater or Leo. This is 132 degrees away from Cygnus. The Sun's gravitational field dominates forces of surrounding stars out to about two light-years equaling 125,000 AU. Lower estimates for radius of Oort cloud do not place it farther than 50,000 AU. Most mass orbits in region between 3,000 and 100,000 AU. Furthest known objects such as Comet West have aphelia around 70,000 AU from Sun. Hill sphere calculated by G.A. Chebotarev extends 230,000 AU.
Up Next
Continue Browsing
Common questions
When did the Sun form and what triggered its creation?
The Sun formed about 4.6 billion years ago from the collapse of part of a giant molecular cloud that consisted mostly of hydrogen and helium. A shock wave from a nearby supernova likely triggered this formation by compressing matter within the cloud.
How long will the Sun remain stable before it becomes a red giant?
The Sun is now roughly halfway through its main-sequence stage and has not changed dramatically in over four billion years. It will remain fairly stable for about five billion more years before expanding into a subgiant and then into a red giant.
What are the physical dimensions and temperature of the Sun's core?
The core of the Sun extends from the centre to about 20% or 25% of the solar radius with a density of up to 150 grams per cubic centimeter. A temperature close to 15.7 million kelvin exists within this region while the surface temperature is about 5,800 kelvin.
Who first proposed that the Sun is the center around which planets orbit?
Aristarchus of Samos proposed the theory that the Sun is the center around which planets orbit in the 3rd century BC. Nicolaus Copernicus later developed a detailed mathematical model of this heliocentric system in the 16th century.
How many times has the Sun orbited the center of the Milky Way galaxy since its formation?
It takes about 220 to 250 million Earth years to complete one revolution known as a galactic year. The Sun has done so approximately 20 times since its formation 4.6 billion years ago.